Raonic dropped the first set but reversed his momentum to score a 3-6, 6-1, 6-2, 7-6 (0) victory over Jan Hajek of the Czech Republic to advance to the second round Tuesday.
The 13th-ranked Thornhill, Ont., native used his power serve to bomb Hajek with 30 aces and broke serve four times from 15 chances, ending with 57 winners and 50 unforced errors.
But he wasn't particularly pleased with his effort.
"It was not my finest two and a half hours," said Raonic. "I wasn't putting anything on the ball the whole match, I was playing a little bit tight. I wasn't hitting freely. I was just letting him get in the points and get in the match."
Raonic also dominated the fourth-set tiebreaker, winning on the first of six match points in windy conditions he said weren't a factor.
"The wind wasn't that bad, as bad as it seemed, it was actually pretty OK to play. I just wasn't swinging freely, and then just the wind seem a lot worse than it is," he said.
"My legs weren't moving, my arm wasn't moving, so the margin for error when I was swinging through the ball was much smaller. But it was the first match. I'm just looking to get better and better."
Later, Ottawa's Jesse Levine outslugged Spain's Tommy Robredo, a former world No. 5, 7-6 (5), 6-7 (2), 6-4, 6-4 in a match that went just over three hours. Levine, playing in his first Grand Slam as a Canadian after previously representing the United States, will face 12th-seeded Gilles Simon of France in the second round.
"You are always happy to win any match in a Slam," said Levine, who said he made his change of nationality because he felt at age 25 that the time was right.
"The first two sets were tiebreakers against a tough player. I'm happy I played really well.
"I'm match-tough, I qualified at both Brisbane and Auckland and won two rounds in Auckland. That might be an advantage. I'm hoping to roll with the positives and stay sharp."
Earlier, Rebecca Marino made a losing start to the season after being eliminated 6-3, 6-0 in the first round of women's play by China's Peng Shuai.
The match was the first in a main draw this season for the 22-year-old Vancouver native, who took more than half a year off in 2012 to get her focus back.
Marino returned to tennis on the Challenger level in October where she won a title, but has failed to qualify for two events so far in 2013, in Auckland and Hobart, Tasmania.
"This is my first grand slam back, and you could tell I was rusty," said Marino, who said had she not taken time away from the game she wouldn't have been on the court Tuesday. "I had a lot of nerves at the start — jelly legs.
"(Peng) is also an extremely good player in the top 40. I was frustrated to lose but I'm not frustrated off the court. I thought I'd be able to squeak back in the end but it didn't happen. I need more match experience."
The match with Peng went quickly for Marino, who lost in 52 minutes. She was broken in the opening game and never found her way into the contest.
Marino saved a match point in the last game after putting late pressure on her opponent with a missed break-point opportunity as Peng served for victory.
Marino exited the Open for the second year in the first round after winning her only main draw match at the tournament in 2011.
Marino ended with four aces, 10 winners and 23 unforced errors.
"I (don't have) big expectations for this season," said Marino, ranked 426th in the world. "I want to get back to the level where I was.
"But I'm not there yet, I'm the first to admit that, I'm not delusional. I'd like to be competing in the top 100 by the end of the year, I think that is possible. But it's baby steps right now."